Palpitations with Anxiety

To start, I am a 24 year old female. For the last two and a half to three months I have been suffering from palpitations. Odd thing though is that they are way worse at night. Some of them feel like they skip so hard and often that it makes me very anxious. I had 2 very short episodes of SVT while at work. The SVT wasnt caught on an EKG, but based on the symptoms I explained to my doctor, he said that it clinically was SVT. I then underwent an Echo to check for Mitral Valve Prolapse, that was normal.I wore a 24 hour monitor that showed no SVT, but that I had roughly 3200 PVCs in a 24 hour period.  I had a standard treadmill stress test as well. When I did the stress test, by baseline EKG showed my resting heart rate to be 133bpm. My target rate during the test was 196, which was reached 7:50 into the test. I then had an extended recovery period because my heart rate would not fall below 120. After the stress test I was put on Metoprolol Succinate 25mg nightly. I started taking the Metorpolol the same day, and for about 2 weeks or so I felt almost completely better. I then wore a 30 event monitor that again showed no arrythmias, and any recording I made just showed palpiations. Sometimes these "attacks" as I describe them keep me up all night. Last night I had chest pain. My throat felt tight, my chest was heavy and I had some pain going into my left shoulder. The pain went away after a few minutes but the pressure lasted for a while. My doctor who did the echo/holter/30 day monitor/stress test kind of chalked everything up to as possibly being a metabolic issue. I do have Hashimotos disease, but I had my thyroid levels checked when I first started having this problem and they were normal. I also have a history of Vitamin B12 and D defiency, but I had that checked when my thyroid was checked and those levels were normal as well. The only thing "abnormal" was that my RBC was minimally elevated.  I dont know what to do anymore. Has anyone else experienced this? Thanks